This invention relates to a contact printing process.
The fabrication of articles such as printed circuit boards has generally included a contact printing operation where a mask is placed in direct contact with a photoresist layer on the board, the structure is exposed to collimated light, and the photoresist is developed to produce an appropriate pattern for further processing. A typical prior art technique placed the article in an envelope which was then evacuated so that outside pressure would force the mask and photoresist into contact. This process can produce registration difficulties when mechanized alignment techniques are used. An alternative approach, which appears to be more compatible with mechanized alignment pins, is known as the "soft contact" method, and relies simply on the weight of the glass frame holding the mask to maintain contact between the mask and photoresist.
Unfortunately, masks develop surface abrasions, and the circuit boards are not consistently planar resulting in less than intimate contact with the mask. Both of these effects tend to blur developed features as a result of diffusion of the collimated light. Thus, it is difficult for the prior art techniques to achieve the resolution capability needed for increasingly finer lines called for in new generations of circuit boards (typically 50.8 to 76.2 microns line widths) at an acceptable yield.
It is, therefore, an object of the invention to improve the resolution capability and yield of a contact printing operation.